19-1372; Rev 1; 6/98 KIT ATION EVALU E L B A AVAIL Low-Noise Bias Supply in µMAX with Power-OK for GaAsFET PA The MAX881R low-noise, inverting power supply is designed for biasing GaAsFET power amplifiers in portable wireless applications. This device is a chargepump inverter followed by a negative linear regulator. The input voltage range is 2.5V to 5.5V. The output is preset at -2.0V or can be set, using two resistors, to any voltage from -0.5V to (-VIN + 0.6V). It can deliver up to 4mA. The internal linear regulator also filters the output to 1mVp-p ripple and noise. Other features include a power-OK (POK) output that signals when the negative voltage is within 7.5% of its set point. It protects the GaAsFET by not allowing power to be applied to the GaAsFET’s drain until it is properly biased. The signal can be routed either to a microcontroller or directly to a switch at the GaAsFET drain. The MAX881R is available in a space-saving 10-pin µMAX package. Features ♦ Small µMAX Package ♦ 1mVp-p Output Voltage Ripple and Noise ♦ Power-OK Signal to Control GaAsFET Drain Switch ♦ 0.05µA Logic-Controlled Shutdown ♦ 1ms Guaranteed Start-Up ♦ 2.5V to 5.5V Input ♦ -0.5V to (-VIN + 0.6V) Output at up to 4mA ♦ Operates with One 4.7µF and Three 0.22µF Capacitors (no inductors needed) Applications Cell Phones Ordering Information PCS Phones PHS Phones PART MAX881REUB Wireless Handsets TEMP. RANGE -40°C to +85°C PIN-PACKAGE 10 µMAX Wireless Modems Two-Way Pagers Mobile Radios Wireless Computers Typical Operating Circuit Pin Configuration TOP VIEW OUTPUT -2V (OR ADJ) 4mA, 1mVp-p RIPPLE INPUT 2.5V TO 5.5V IN OUT C1+ 1 MAX881R ON OFF C1- 2 NEGOUT 3 POK 4 7 OUT SHDN 5 6 FB SHDN C1+ POK C1NEGOUT FB GND CONTROLS GaAsFET PA’s DRAIN SWITCH 10 IN MAX881R 9 N.C. 8 GND µMAX N.C. = NOT INTERNALLY CONNECTED ________________________________________________________________ Maxim Integrated Products 1 For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. For small orders, phone 408-737-7600 ext. 3468. MAX881R General Description MAX881R Low-Noise Bias Supply in µMAX with Power-OK for GaAsFET PA ABSOLUTE MAXIMUM RATINGS IN to GND .................................................................-0.3V to +6V SHDN to GND...........................................................-0.3V to +6V POK to GND ...........................................................-0.3V to +12V C1+ to GND .................................................-0.3V to (VIN + 0.3V) C1-, NEGOUT, OUT, FB to GND ....................-6V to (VIN + 0.3V) Continuous Power Dissipation (TA = +70°C) 10-Pin µMAX (derate 5.6mW/°C above +70°C) ...........444mW Operating Temperature Range ...........................-40°C to +85°C Junction Temperature ......................................................+150°C Storage Temperature Range .............................-65°C to +165°C Lead Temperature (soldering, 10sec) .............................+300°C Note 1: The output may be shorted to NEGOUT or GND if the package power dissipation is not exceeded. Typical short-circuit current is 35mA. Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS (Circuit of Figure 3, VIN = +3.6V, FB = GND, RL = ∞, SHDN = IN, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 2) PARAMETER SYMBOL CONDITIONS Supply Voltage Range VIN Preset Output Voltage VOUT VIN ≥ 2.7V, IOUT = 0 to 4mA Adjustable Output Voltage Range VOUT VIN ≥ 2.5V, IOUT = 0 to 4mA FB Voltage VFB FB Input Current Supply Current Shutdown Supply Current MIN VIN ≥ 2.5V, IOUT = 0 to 4mA -2.1 -2.0 -(VIN - 0.6) TA = +25°C -0.515 TA = 0°C to +85°C -0.525 TA = -40°C to +85°C -0.535 VFB = -0.5V IQ SHDN = GND -0.5 MAX UNITS 5.5 V -1.9 V -0.5 V -0.485 -0.475 V -0.465 -10 -100 nA 500 950 µA 0.05 1 µA Output Load Regulation VIN ≥ 2.7V, IOUT = 0 to 4mA 2 6 mV/mA Output Ripple IOUT = 4mA, circuit of Figure 3b 1 Oscillator Frequency ISHUT TYP 2.5 fOSC 80 POK Threshold FB = OUT POK Output Level VIN ≥ 2.5V, sinking 1mA POK Off Leakage Current VIH VIN = 5.5V SHDN Input Low Voltage VIL VIN = 2.5V SHDN Input Capacitance Start-Up Time 92.5 V POK = 11V SHDN Input High Voltage SHDN Input Current 90 100 ISHDN tSTART kHz 95 % of VOUT 100 mV 1 µA V 0.35 V ±1 µA 10 VIN = 3V, RL = 500Ω, V SHDN = 0 to VIN, POK goes low Note 2: Specifications to -40°C are guaranteed by design, not production tested. 2 120 2.2 Connected to IN or GND CIN mVp-p _______________________________________________________________________________________ pF 1 ms Low-Noise Bias Supply in µMAX with Power-OK for GaAsFET PA 1.90 -1.92 -1.94 VIN = 2.5V -2.00 -1.90 -1.92 -1.96 1 2 3 4 5 6 7 8 9 VIN ≥ 3.6V -1.98 -1.99 -2.00 -2.02 2 4 6 8 10 12 14 2.0 16 2.5 3.0 3.5 4.0 4.5 5.0 5.5 OUTPUT CURRENT (mA) OUTPUT CURRENT (mA) INPUT VOLTAGE (V) MAXIMUM OUTPUT CURRENT vs. INPUT VOLTAGE MAXIMUM NEGOUT CURRENT vs. INPUT VOLTAGE NO-LOAD SUPPLY CURRENT vs. INPUT VOLTAGE VOUT < 2mVp-p RIPPLE CIRCUIT OF FIGURE 3b 20 15 10 VOUT < 2mVp-p RIPPLE CIRCUIT OF FIGURE 3a 5 16 12 8 0 800 CIRCUIT OF FIGURE 3a 4 3.0 3.5 4.0 4.5 5.0 5.5 6.0 700 600 500 400 0 2.5 MAX881R TOC06 MAX881R TOC05 CIRCUIT OF FIGURE 3b 300 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 2.0 2.5 3.0 3.5 4.0 4.5 5.0 INPUT VOLTAGE (V) INPUT VOLTAGE (V) INPUT VOLTAGE (V) NO-LOAD SUPPLY CURRENT vs. TEMPERATURE START-UP TIME vs. INPUT VOLTAGE OSCILLATOR FREQUENCY vs. INPUT VOLTAGE 600 500 VIN = 3.6V 400 600 500 400 300 6.0 MAX881R TOC09 700 5.5 103 TA = -40°C 102 FREQUENCY (kHz) 700 CIRCUIT OF FIGURE 3a NO LOAD SHDN TO POK DELAY MAX881R TOC08 VIN = 5.5V START-UP TIME (µs) 800 800 MAX881R TOC07 900 6.0 900 SUPPLY CURRENT (µA) VOUT ≤ -1.8V CIRCUIT OF FIGURE 3a 25 VNEGOUT = 0.9 x VNEGOUT(NO LOAD) 20 OUTPUT CURRENT (mA) 35 30 24 MAX881r TOC04 VOUT ≤ -1.8V CIRCUIT OF FIGURE 3b 2.0 MAX881R TOC03 -1.97 -2.03 0 10 ILOAD = 4mA -1.96 -2.01 -2.02 0 SUPPLY CURRENT (µA) VIN = 2.5V -1.94 -2.00 VIN ≥ 3.6V -2.02 OUTPUT CURRENT (mA) -1.88 -1.98 -1.98 40 -1.86 OUTPUT VOLTAGE (V) OUTPUT VOLTAGE (V) -1.88 CIRCUIT OF FIGURE 3b OUTPUT VOLTAGE (V) -1.86 -1.95 MAX881R TOC02 CIRCUIT OF FIGURE 3a 45 -1.84 MAX881R TOC01 -1.84 -1.96 OUTPUT VOLTAGE vs. INPUT VOLTAGE OUTPUT VOLTAGE vs. OUTPUT CURRENT OUTPUT VOLTAGE vs. OUTPUT CURRENT TA = +25°C TA = +85°C 101 100 99 98 200 VIN = 2.5V 100 300 -60 -40 -20 0 20 40 TEMPERATURE (°C) 60 80 100 97 2.0 2.5 3.0 3.5 4.0 4.5 INPUT VOLTAGE (V) 5.0 5.5 6.0 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 INPUT VOLTAGE (V) _______________________________________________________________________________________ 3 MAX881R Typical Operating Characteristics (Circuit of Figure 3, VIN = 3.6V, TA = +25°C, unless otherwise noted.) Typical Operating Characteristics (continued) (Circuit of Figure 3, VIN = 3.6V, TA = +25°C, unless otherwise noted.) OUTPUT NOISE AND RIPPLE (CIRCUIT OF FIGURE 3a) START-UP FROM SHUTDOWN (NO LOAD) 0 VOUT 0 -2V POK MAX881R-TOC12 VIN = 3.6V VOUT = -2V IOUT = 2.5mA +5V SHDN OUTPUT NOISE AND RIPPLE (CIRCUIT OF FIGURE 3b) MAX881R-TOC11 MAX881R-TOC10 RL = 500Ω VOUT = 0 VIN = 0 1mV/div AC COUPLED VOUT 1mV/div AC COUPLED 10µs/div OUTPUT NOISE SPECTRUM (CIRCUIT OF FIGURE 3a) OUTPUT NOISE SPECTRUM (CIRCUIT OF FIGURE 3b) VIN = 3.6V VOUT = -2V IOUT = 2.5mA VIN = 3.6V VOUT = -2V IOUT = 4mA 50 40 30 MAX881R TOC14 60 MAX881R TOC13 60 40 OUTPUT NOISE 5µs/div 100µs/div 50 NOISE FLOOR VOUT = -2V VIN = 3.6V 1mV/div AC COUPLED VIN 0 VIN = 3.6V CIRCUIT OF FIGURE 3b 30 NOISE (dBµV) NOISE (dBµV) MAX881R Low-Noise Bias Supply in µMAX with Power-OK for GaAsFET PA 20 10 0 -10 20 10 0 -10 -20 -20 -30 -30 -40 -40 1 10 100 1000 1 FREQUENCY (kHz) 10 100 1000 FREQUENCY (kHz) LINE-TRANSIENT RESPONSE LOAD-TRANSIENT RESPONSE MAX881R-TOC15 MAX881R-TOC16 4mA 3.3V IOUT VIN 400µA 2.7V 10mV/ div VOUT ILOAD = 4mA VOUT = -2V CIRCUIT OF FIGURE 3b 2ms/div 4 VOUT VIN = 3.6V VOUT = -2V CIRCUIT OF FIGURE 3b 100µs/div _______________________________________________________________________________________ 10mV/ div Low-Noise Bias Supply in µMAX with Power-OK for GaAsFET PA PIN NAME FUNCTION 1 C1+ Positive Terminal for C1 2 C1- Negative Terminal for C1 3 NEGOUT 4 POK 5 SHDN Active-Low, Logic-Level Shutdown Input. Connect to IN for normal operation. Do not leave this pin unconnected. 6 FB Dual-Mode™ Feedback Input. When FB is connected to GND, the output is preset to -2V. To select other voltages, connect FB to an external resistor-divider (Figure 4). Do not leave this pin unconnected. 7 OUT Regulated Negative Output Voltage 8 GND Ground 9 N.C. No Connection. Not internally connected. 10 IN Negative Output Voltage (unregulated) Active-Low, Open-Drain Power-OK Output. Goes low when OUT reaches 92.5% of its set value. Positive Power-Supply Input Dual-Mode is a trademark of Maxim Integrated Products. SHDN C1+ C1 C1- IN CHARGE PUMP C3 NEGOUT OUT C4 C2 FB MAX881R 0.5V POK 0.4625V GND Figure 1. Functional Diagram _______________________________________________________________________________________ 5 MAX881R Pin Description MAX881R Low-Noise Bias Supply in µMAX with Power-OK for GaAsFET PA TO PA GATE BIAS OUT VIN VCC FOR POWER AMPLIFIER C3 0.22µF C1+ C4 MAX881R 100k POK IN OUT C1 0.22µF P OFF TO POWER AMPLIFIER MAX881R C1- ON VOUT = -2V C4 4.7µF SHDN NEGOUT C2 0.22µF POK GND FB Figure 2. Using the POK Function ________________Detailed Description The MAX881R, a low-noise, inverting, regulated chargepump power supply, is designed for biasing GaAsFET devices such as power-amplifier modules in cellular handsets. The applied input voltage (VIN) is inverted to a negative voltage at NEGOUT by a capacitive charge pump. This voltage is regulated by an internal linear regulator at OUT (Figure 1). With FB connected to GND, VOUT is regulated at -2V. Alternatively, use a voltage-divider at FB to adjust the output voltage between -0.5V and -(VIN - 0.6V) (see the section Setting the Output Voltage). The internal linear regulator reduces the ripple noise induced by the charge-pump inverter to 1mVp-p at OUT (circuit of Figure 3b). In addition, the excellent AC rejection of the linear regulator attenuates noise from the incoming supply. Power-OK Signal The MAX881R has an active-low, open-drain, power-OK (POK) output. This output goes low when OUT reaches 92.5% of the regulated output voltage. POK can be used to drive the gate of a P-MOSFET that switches power to the GaAsFET power amplifier (Figure 2), thereby ensuring that the power amplifier is not powered before the required negative bias voltage is present. Use a 50kΩ or larger pull-up resistor to signal a logic high when POK goes high impedance. Shutdown Mode The MAX881R features a shutdown mode that reduces supply current to less than 1µA over temperature. SHDN is an active-low, logic-level input. Start-up time coming out of shutdown mode is typically 0.5ms. OUT and NEGOUT are switched to GND in shutdown mode. 6 Figure 3a. Standard Application Circuit for Minimum Capacitor Values VIN C3 1µF C1+ IN OUT C1 1µF MAX881R C1- ON OFF SHDN NEGOUT C2 1µF VOUT = -2V C4 10µF POK GND FB Figure 3b. Standard Application Circuit for Minimum Output Noise Applications Information Setting the Output Voltage Select either a fixed or adjustable output for the MAX881R. Connect FB to GND for a fixed -2V output (Figure 3). Select an alternative output voltage by connecting FB to the midpoint of a resistor-divider from OUT to GND (Figure 4). When operating under full load (4mA), the voltage at IN should be at least 0.6V higher than the absolute voltage required at OUT. Note that the minimum input voltage required for operation is 2.5V, regardless of the desired output voltage. Choose R1 to be between 100kΩ and 400kΩ and calculate R2. For greater accuracy, use resistors with 1% or better tolerance. R2 = R1 (2 x |VOUT| - 1) _______________________________________________________________________________________ Low-Noise Bias Supply in µMAX with Power-OK for GaAsFET PA C3 0.22µF IN C1+ C1 0.22µF OFF C2 0.22µF R2 MAX881R C1- ON OUT C4 4.7µF FB SHDN NEGOUT POK R1 GND C1, C2, and C3 should be 0.22µF capacitors with less than 0.4Ω ESR. C4 should be a 4.7µF capacitor with less than 0.1Ω ESR. Larger capacitor values can be used (C1 = C2 = C3 = 1µF, C4 = 10µF) to reduce output noise and ripple (1mVp-p), at the expense of cost and board space. All capacitors should be either ceramic or surface-mount chip tantalum (Figures 3a and 3b). Layout and Grounding Good layout is important for good noise performance. To optimize the layout: Figure 4. Adjustable Output Configuration Capacitors Use capacitors with a low effective series resistance (ESR) to maintain a low dropout voltage (VIN - |VOUT|). The overall dropout voltage is a function of the output resistance of the charge pump and the voltage drop across the linear regulator (N-channel pass transistor). At the 100kHz charge-pump switching frequency, output resistance is a function of the value of C1 and the ESR of C1 and C2. Therefore, increasing C1 and minimizing the ESR of the charge-pump capacitors minimizes dropout voltage. The output resistance of the entire circuit (in dropout) is approximately: ROUT = RO + 4 x ESRC1 + ESRC2 + 1 / (fS x C1) + R(linear regulator) 1) Mount all components as close together as possible. 2) Keep traces short to minimize parasitic inductance and capacitance, especially connections to FB. 3) Use a ground plane. Noise and Ripple Measurement Accurately measuring the output noise and ripple is a challenge. Slight momentary differences in ground potential between the circuit and the oscilloscope (which result from the charge pump’s switching action) cause ground currents in the probes’ wires, inducing sharp voltage spikes. For best results, measure directly across the output capacitor (C4). Do not use the ground lead of the probe; instead, remove the probe’s tip cover and touch the ground ring on the probe directly to C4’s ground terminal. This direct connection gives the most accurate noise and ripple measurement. Where R(linear regulator) (the output impedance of the linear regulator) is approximately 2Ω and RO (the resis- Chip Information TRANSISTOR COUNT: 413 _______________________________________________________________________________________ 7 MAX881R tance of the internal switches) is typically 10Ω. When regulating, the output resistance of the circuit is simply the linear-regulator load regulation (2mV/mA). VIN Low-Noise Bias Supply in µMAX with Power-OK for GaAsFET PA 10LUMAXB.EPS MAX881R Package Information 8 _______________________________________________________________________________________